1. Field of the Invention
The present invention relates to a large-capacity inverter suitable for use with a self-extinguishing type semiconductor element, and more particularly, to an energy-loss reduction technique for reducing to as low as possible an energy loss within a large-capacity inverter.
2. Description of the Related Art
A large-capacity self-arc-extinguishing semiconductor element is currently employed in a large-capacity inverter. Examples of the self-arc-extinguishing semiconductor element include a gate-commutated turn-off thyristor (GCT) and an insulated gate bipolar transistor (IGBT) The GCT immediately transfers all conduction currents to a gate circuit and performs a turn-off operation at a turn-off gain of 1. The GCT is larger in capacity than the IGBT. Currently, the IGBT has a maximum rating of about 4.5 kV and about 1.5 kA. As of now, a GCT has a rating of 6.0 kV and 6.0 kA, as a result of application of a 6-inch silicon wafer to the GCT. Continued improvements in ratings of the IGBT and GCT are expected in the future.
FIG. 15 shows a conventional inverter to which the GCT is applied, particularly showing the configuration of a two-level inverter bridge. FIG. 15 is disclosed in Japanese Patent Unexamined Publication No. Hei. 9-182460. In FIG. 15, reference numeral 1 designates a d.c. voltage circuit (having voltage E) having potentials P and N; 2a and 2b designate GCTs acting as self-arc-extinguishing elements; 3a and 3b designate free-wheeling diodes; 4 designates an anode reactor; 5 designates a clamping diode; 6 designates a clamping capacitor; and 7 designates a discharge resistor. Symbol "OUT " designates an output terminal connected to a load. The rate of increase in critical voltage of the GCTs 2a and 2b is several times that of a conventional gate turn-off thyristor. Since the GCTs 2a and 2b have a wide safety operation range, the inverter does not require use of a charge-and-discharge snubber circuit, which would otherwise be required when gate turn-off thyristors are employed. For this reason, a voltage clamping circuit shown in FIG. 15 can be applied to the inverter.
For instance, during the period of turn-on operation of the GCT 2a, the energy induced by a reverse recovery current output from the free-wheeling diode 3b is stored in the anode reactor 4. Further, during the period of turn-off operation of the GCT 2a, the energy induced by a load current is stored in the anode reactor 4. After having been temporarily stored in the clamping capacitor 6, the thus-stored energy is dissipated by the discharge resistor 7. The maximum level of the reverse recovery current which develops in the free-wheeling diode 3b as a result of the turn-on operation of the GCT 2a greatly depends on the magnitude of the load current, the rate of change in the current determined on the basis of the magnitude E of the voltage of the d.c. voltage circuit 1 and the inductance of the anode reactor 4, or the virtual junction temperature of the free-wheeling diode 3b.
Next will be described an abrupt change in the voltage E of the d.c. voltage circuit 1 in the circuit shown in FIG. 15, particularly a rise in the voltage E of the d.c. voltage circuit 1. In the circuit shown in FIG. 15, the clamping capacitor 6 is recharged by way of the anode reactor 4 and the clamping diode 5, to thereby diminish a difference between the voltage of the d.c. voltage circuit 1 and the voltage of the clamping capacitor 6. The reason for this recharge is that a reverse voltage is not applied to the clamping diode 5 except when the GCTs 2a and 2b are in a transitory phase from one switching operation to another switching operation.
Incidentally, market demand exists for an increase in the capacity of an inverter. The rated capacity of the inverter shown in FIG. 15 may be increased by means of a three-level inverter. If the inverter shown in FIG. 15 is expanded to a three-level inverter bridge, the resultant circuit may assume a configuration such as that shown in FIG. 16. Such a configuration is not schematically shown in the above-described Japanese Patent Unexamined Publication No. Hei. 9-182460. Potential P and potential C are applied to the d.c. voltage circuit 1a, and potential C and potential N are applied to the d.c. voltage circuit 1b. A difference between potential P and potential C is indicated by E and is equal to a difference between potential C and potential N. Reference numerals 8a and 8b designate coupling diodes. In contrast with the voltage which can be output from the output terminal OUT of the circuit shown in FIG. 15, a voltage 2E can be output from the output terminal OUT of the circuit shown in FIG. 16. If GCTs of the same rated capacity are applied to the inverter shown in FIG. 15 and that shown in FIG. 16, a comparison between the inverters would show that the rated capacity of the inverter shown in FIG. 16 is twice that of the inverter shown in FIG. 15. Here, the inverter shown in FIG. 16 must have two anode reactors 4a and 4b.
In a case where the capacity of an inverter is increased in order to meet market demand, a load current or the voltage of the d.c. voltage circuit 1 must be increased, which in turn may increase the reverse recovery current output from the free-wheeling diode 3. Alternatively, in a case where a large-diameter diode is used as the free-wheeling diode 3, an increase in the area of the wafer causes an increase in the area of a junction. Accordingly, the reverse recovery current is increased further. The energy stored in the anode reactor 4 is proportional to the square of the current flowing through the anode reactor 4, and hence the power loss, which would be induced by the discharge resistor 7, is significantly increased, thus posing a problem of a decrease in the efficiency of the inverter.
Further, in the event that a gate signal for activating the GCT 2 becomes faulty and a short-circuit arises in the d.c. voltage circuit 1 due to faulty operations of the GCT 2, excessive short-circuit current flows to the main circuit of the inverter shown in FIG. 15. Such a short-circuit current may induce deformation of the anode reactor 4 or deformation or fracture of metal conductors used for interconnecting individual constituent components. Conceivable measures to eliminate such a possibility include increasing the size of the anode reactor 4 and connecting fuses in series. In a case where consideration is given to the surge current withstand of the GCT 2 and the pre-arcing time-current characteristic of a fuse, there arises a problem of a necessity for increasing the inductance of the anode reactor 4. If the inductance of the anode rector 4 is increased, the power loss caused by the discharge resistor 7 is increased to a much greater extent. Eventually, there arises a problem of an increase in the power loss of the inverter and deterioration of efficiency thereof.
In the case of an increase in the d.c. voltage circuit, the clamping capacitor 6 is inevitably recharged by way of the clamping diode 5 in the manner as mentioned above. If the GCT 2 commences a turn-on operation while the charging current is flowing through the clamping diode 5, a current which is characterized in involving an excessive rate of increase flows into the GCT 2 during a period from the time the clamping diode 5 commences a reverse recovery operation until the time the charging voltage applied to the clamping capacitor 6 is reversely applied to the clamping diode 5. This may induces a faulty turn-on operation of the GCT 2 or application of an excessive surge voltage to the clamping diode 5, which would result in breakage of the main circuit element.
In a case where the three-level inverter bridge shown in FIG. 6 is applied to the inverter in order to increase the capacity thereof, the number of anode reactors 4 is increased. Consequently, the power loss induced by the inverter is increased, thereby rendering a cooling device bulky.